Universe and the Solar System

Every question in this chapter rests on one big idea: everything in the sky is held together by gravity, and everything is very, very far away. Before we meet the Sun and the planets, let us learn the full address of our home: Earth, in the Solar System, in the Milky Way, in the Universe.

Universe means all matter, energy, space and time together. Its scientific study is cosmology, while the study of individual bodies in the sky is astronomy. A celestial body is any natural object in space: a star, planet, moon, asteroid, comet, nebula or galaxy. Luminous bodies (stars) make their own light; non-luminous bodies (planets, moons, comets) only shine by reflecting a star's light.

Now the problem of distance. In space, kilometres are too small to be useful, just as you would not measure the distance from Delhi to Chennai in millimetres. So astronomers use three bigger units, each made for a different scale of distance.

How did all this begin? The idea accepted by almost all scientists today is the Big Bang. About 13.8 billion years ago, all matter and energy were packed into an unimaginably hot and dense state, and space itself began to expand. It was not an explosion happening inside empty space; space itself stretched, which is why galaxies move away from one another, like dots on a balloon that is being inflated. As the universe cooled, atoms formed (roughly 3 lakh years after the Big Bang, at a temperature near 4,500 K; later measurements put it nearer 3.8 lakh years), light could finally travel freely, and the universe became transparent. Stars and galaxies formed much later.

Our own Solar System is much younger than the Universe, about 4.6 billion years old. It formed from a huge spinning cloud of gas and dust. The centre of the cloud collapsed to become the Sun; the flattened disc around it clumped together into planets, moons and smaller bodies. Several scientists proposed models for this, and the names are favourite exam material.

Gravity is the glue of the sky. It pulled the cloud together to make the Sun, it keeps the planets in orbit, it holds the Moon around Earth, and it binds billions of stars into a galaxy. Whenever a question asks which force governs motion in space, the answer is gravitation.

Look at the night sky and you see thousands of points of light. Almost every one of them is a star: a gigantic ball of hot gas, mostly hydrogen and helium, that makes its own light and heat. The Sun is simply the star that happens to be close to us. Understanding how stars are born, live and die is the key to this whole topic, and exams love the 'end of the story' part.

A star lives by a tug of war. Gravity tries to crush the star inwards; the energy from nuclear fusion in its core (hydrogen fusing into helium) pushes outwards. While the two are balanced, the star shines steadily. When the fuel runs low, the balance breaks and the star changes. A bigger star burns its fuel faster, so it lives a shorter and more dramatic life.

Every star begins in a nebula, a vast cloud of gas and dust, often called the nursery of stars. Part of the cloud collapses under its own gravity, heats up, and becomes a protostar. When the core becomes hot enough for fusion to start, a true star is born and settles down as a main-sequence star; our Sun is in this stage now. What happens next depends only on the star's mass.

The Chandrasekhar limit is the maximum mass a white dwarf can have: about 1.4 times the mass of the Sun. It was worked out by the Indian-born astrophysicist Subrahmanyan Chandrasekhar (Nobel Prize 1983). A star core heavier than this cannot settle as a white dwarf; it collapses further and ends in a supernova, leaving a neutron star or a black hole.

Stars differ in brightness, colour and distance, and exams mix these up. Brightness as seen from Earth depends on how much light a star gives out and how far it is. Colour shows temperature: blue-white stars are hottest, red stars are coolest, and the Sun, a yellowish-white star, is in between. Stars also twinkle because their light is a single point passing through moving layers of air that keep bending it (refraction); planets look like tiny discs, so these small wobbles average out and they shine steadily.

Billions of stars, along with gas, dust and dark matter, are bound together by gravity into a galaxy. Galaxies are classified by shape, and the classification is exam-friendly. Our own galaxy is the Milky Way (Akash Ganga), a barred spiral about one lakh light-years across; the Sun sits in one of its spiral arms, far from the centre, and circles the galaxy once in roughly 22 to 25 crore years (a 'cosmic year').

The Sun is the centre of our Solar System and the only star close enough to study in detail. It holds about 99.86 per cent of the whole Solar System's mass, which is why every planet, comet and asteroid is forced by its gravity to go around it. It is also the only body in the Solar System that makes its own light, and almost all the energy for life on Earth comes from it.

The Sun shines by nuclear fusion: in its core, hydrogen nuclei are squeezed together so hard that they join to form helium, and a tiny bit of mass turns into a huge amount of energy. Do not confuse this with fission, the splitting of heavy nuclei such as uranium, which is what nuclear power plants use. Sun = fusion; power plant = fission.

Think of the Sun as an onion with six layers. The three inner layers are never seen directly. The three outer layers together make the Sun's atmosphere, and these are the ones you can name from observation.

The Sun is not calm. Its strong magnetic field keeps stirring up activity, and exam questions are mostly about the names and the effects.

India now has its own Sun mission. Aditya-L1, launched on 2 September 2023 from Sriharikota, is India's first dedicated space-based solar observatory. It does not go to the Sun: it sits in a halo orbit around the L1 Lagrange point, about 15 lakh km from Earth towards the Sun, where the pull of the Sun and Earth balance and the spacecraft gets an uninterrupted view of the Sun. It reached its halo orbit on 6 January 2024.

A planet is a large body that goes around the Sun on its own path (an orbit), is massive enough to be round, and has cleared its neighbourhood of other big objects. Our Solar System has eight planets. Think of them as eight runners on eight lanes around one stadium light. Their order from the Sun is: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune. Between Mars and Jupiter runs the asteroid belt, and Pluto is no longer counted as a planet.

The eight planets fall into two families, and almost every comparison question is really asking which family a planet belongs to. The inner (terrestrial) planets are small, rocky and dense, with few or no moons. The outer (Jovian) planets are huge, made mostly of gas and liquid, light for their size, and have rings and many moons.

Among the outer planets there are two sub-families. Jupiter and Saturn are gas giants, made mainly of hydrogen and helium. Uranus and Neptune are ice giants, with much more water, ammonia and methane ices inside, and methane in their air gives them their blue-green colour. None of the four has a solid surface you could stand on, so a statement like 'all Jovian planets have solid surfaces' is false.

Now the planets one by one. First the four inner planets. Each row gives the one identity the examiner loves, followed by the facts that sit around it.

Now the four outer planets. Here the numbers are huge, so focus on the single superlative that each one owns.

Why is Venus hotter than Mercury? Mercury has almost no air, so heat escapes into space at night. Venus is wrapped in a very thick blanket of carbon dioxide (a runaway greenhouse effect), so heat stays trapped day and night, everywhere on the planet. Hottest = Venus (blanket); closest = Mercury (distance). They are different answers to different questions.

What keeps the planets going round the Sun? Gravity pulls each planet towards the Sun, and the planet's own forward motion stops it from falling in. The balance of the two gives a curved path. The paths are ellipses (slightly squashed circles), not perfect circles, and Johannes Kepler summed this up in three laws.

Missions are an easy source of questions because they join planets with current affairs.

Besides the eight planets, the Sun's family has many smaller members: dwarf planets, asteroids, comets and meteoroids. They are the left-overs of the cloud that formed the Solar System about 4.6 billion years ago. Scientists sort them using a few simple tests, so first learn the tests, and then the members will fall into place.

Why was Pluto demoted? Pluto was discovered in 1930 by Clyde Tombaugh and was called the ninth planet for 76 years. In the 1990s and 2000s astronomers found many similar icy worlds beyond Neptune, and one of them, Eris, was nearly as large as Pluto. So in August 2006 the International Astronomical Union (IAU), meeting in Prague, wrote the three-part definition of a planet. Pluto passes the first two tests but fails the third: it shares its orbit with many other icy bodies in the Kuiper belt. It became a dwarf planet.

Between the orbits of Mars and Jupiter lies the asteroid belt, a wide ring of millions of rocky and metallic lumps. Jupiter's strong gravity kept these pieces from joining into a planet, so they stayed as debris. All together they weigh much less than our Moon. Asteroids are rocky lumps, from tiny boulders to hundreds of kilometres across; they are not all in the belt, since some cross Earth's path (near-Earth asteroids), and the 'Trojan' asteroids share Jupiter's orbit.

A comet is a ball of ice, dust and frozen gas, often called a 'dirty snowball', that travels in a long oval orbit around the Sun. When it comes near the Sun, the ice turns to gas and a glowing cloud called the coma forms around the solid nucleus. The Sun's light and the solar wind then push the gas and dust into a long tail.

Now the small rocks that fall towards Earth. The same object gets three different names depending on where it is.

When Earth passes through the dusty trail left by a comet, many meteors appear in a short time, called a meteor shower; examples are the Perseids (August) and the Leonids (November). A large meteorite can dig a bowl-shaped impact crater. In India, Lonar Lake in Buldhana district, Maharashtra, is a lake in a crater made by a meteorite impact about 50,000 years ago, and it is a Ramsar site. A far bigger impact about 66 million years ago, whose crater lies at Chicxulub in Mexico, is linked with the end of the dinosaurs.

The Moon is Earth's only natural satellite, about 3.84 lakh km away and a little over one-quarter of Earth's width. It makes no light of its own; the glow we call moonlight is sunlight bounced off its dusty surface, and it takes only about 1.3 seconds to reach us. Its gravity is just one-sixth of Earth's, which is why astronauts bounce along on it. That weak gravity also explains its biggest oddity: the Moon cannot hold gas around itself, so it has no atmosphere, no wind, no weather and no sound. The sky is black even at noon, and the surface swings from over 100 degrees Celsius in the lunar day to below minus 150 degrees at night.

Why do we always see the same face of the Moon? The Moon spins on its axis exactly once in the same time that it takes to go once around Earth (about 27.3 days). This match is called synchronous rotation. So the Moon does rotate; it just rotates at the same rate as it revolves, and the 'far side' stays hidden from us. The far side is not 'dark'; it gets sunlight as much as the near side.

The Moon's shape in the sky changes every night. This is not Earth's shadow; it is a matter of how much of the Moon's sunlit half we can see. Half of the Moon is always lit by the Sun. As the Moon goes around Earth, we look at that lit half from a changing angle. When the Moon is between us and the Sun, its lit side faces away from us (new moon, Amavasya). When Earth is between the Sun and Moon, we see the whole lit face (full moon, Purnima).

An eclipse happens when the Sun, Earth and Moon line up in a straight line (a position called syzygy) so that one body's shadow falls on another. The body in the middle casts the shadow. Everything else follows from this one sentence.

Three kinds of solar eclipse. The Moon's distance from Earth changes. When it is near, it looks big enough to cover the Sun fully: a total eclipse, and the Sun's corona becomes visible. When it is far, it looks slightly smaller than the Sun and leaves a bright ring around its edge: an annular eclipse (the 'ring of fire'). When only part of the Sun is covered, it is a partial eclipse.

Tides are the regular rise and fall of the sea. The cause is gravity. The Moon pulls on Earth, and the oceans, being free to move, bulge up towards the Moon and also on the opposite side, making two high tides and two low tides in a little over a day (the gap between two high tides is about 12 hours 25 minutes). The Sun also pulls, but because it is so far away its effect is about half of the Moon's.

Why tides matter. Tides help ships enter shallow harbours at high water, and tidal energy is a renewable source that depends on the Moon's pull. The world's highest tides are in the Bay of Fundy (Canada). In India the biggest tidal ranges are in Gujarat's Gulf of Khambhat and Gulf of Kachchh, and Kandla is a tidal port.